In nature, white-rot fungi, known lignin degraders, use plant matter as a source of nutrients, whereas the majority of microorganisms that live on plant matter do not degrade lignin. It is believed that lignin is a precursor of low-rank coals. A number of white-rot fungi are able to degrade low-rank coals as reported in the literature. See Cohen, et al., Applied Environmental Microbiology, 44: 23-27 (1982); Wilson, et al., Proceedings of the Tenth Annual EPRI Contractors Conference on Coal Liquefaction, May 6, 1986, Palo Alto, CA. Scott, et al., Biotechnology Progress, 2, 131-139 (1986); and Cohen, et al., Proceedings of the Direct Liquefaction Contractors Meeting, U.S. Department of Energy: Washington, D.C.; pages IV-48 to IV-64 (1986). The process of solubilizing low-rank coal with white-rot fungi has been termed "biosolubilization", indicating the metabolic origin of the soluble product. Work on biomineralization using German hard coals has been reported by Fakausa, Ph.D. thesis, Fredrich-Wilhelms University, Bonn, Federal Republic of Germany (1981); and the literature of bioconversion of coals, its relation to lignin degradation and work of lignin has been reviewed by Pyne and Wilson; Biological Coal Beneficiation Literature Review, Battelle Pacific Northwest Laboratory Report to the Electric Power Research Institute, May 1986.
Pyne et al., Appl. Environ. Microbiol. 53: 2844-48 (1987), have shown that an extracellular product from C. versicolor can biosolubilize leonardite and that this product was produced in conjunction with an oxidative enzyme, laccase (syringaldezine oxidase), which is also produced by the fungus. Initial purification of the extracellular product by ultrafiltration procedures indicated the product molecular weight is between 10,000 and 100,000, as disclosed in commonly assigned copending Ser. No. 69,709, filed July 6, 1987.
We have now found that the major portion of the coal solubilizing activity in the cell-free filtrates of C. versicolor is contained in the low molecular weight fractions of less than 5000 daltons, and particularly in the range of 500-1000 daltons. We have also found that the active coal solubilizing materials are likely to be siderophore-like which solubilize coal by a chelation mechanism.
It is therefore an object of the present invention to provide a method for producing low molecular weight cell-free filtrates of C. versicolor which are capable of biosolubilizing low-rank coals.
It is a further object of the present invention to provide a method for biosolubilizing low-rank coals to a soluble product using low molecular weight cell-free filtrates of C. versicolor.
These and other objects of the present invention will be readily apparent from the following description, appended claims and by practice of the invention described herein.